1. Field of the Invention
The present invention relates to a capacitance measurement device for a touch control device, and more particularly, to a capacitance measurement device that precisely measures the capacitance of a measured capacitor in a touch control device.
2. Description of the Prior Art
A touchscreen is an LCD screen combined with a touch panel, widely applied in a variety of consumer electronics as a user interface. A projected capacitive touch technology permits higher sensibility, durability and multi-touch operation and is popularly used in touch panels. Please refer to FIG. 1, which is a schematic diagram of a touch control device 10 according to the prior art. The touch control device 10 comprises a touch panel 100, a multiplexer 102, a capacitance measurement device 104, a processing unit 106, and a memory 108. The touch panel 100 is a projected capacitive touch panel consisting of intersecting Indium Tin Oxide (ITO) traces that act as row and column electrodes. Each trace is equivalent to an RC circuit composed of a resistor and a capacitor. When a user touches or approaches the touch panel 100, a human body capacitor may be coupled to the touched trace and thus the capacitance of the trace changes. In other words, the touched trace is regarded as a measured capacitor for the capacitance measurement device 104. The multiplexer 102 is coupled to all traces of the touch panel 100 and is utilized for conducting a connection between each trace and the capacitance measurement device 104. In other words, the capacitance measurement device 104 scans the touch panel 100 through the multiplexer 102 for detecting if a touch happens. The capacitance measurement device 104 converts the capacitance of the measured capacitor into a recordable value as an analog voltage or a digital count value, outputted to the processing unit 106.
When the touch panel 100 is not touched, the capacitor of each trace is regarded as an environment capacitor. The capacitance of the environment capacitor is also measured and converted into a base count value by the capacitance measurement device 104, and is stored in the memory 108. Touch panels of different characteristics may have different capacitance of the environment capacitor. Whether the measured capacitance of the environment capacitor is accurate influences touch detection. When the touch panel 100 is touched, a human body capacitor is coupled to the measured capacitor and the capacitance of the measured capacitor changes. The processing unit 106 compares a new count value generated by the capacitance measurement device 104 with the base count value and thereby determines if the touch panel 100 is touched.
There are several ways for the capacitance measurement device 104 to measure the capacitance of the measured capacitor. A simple way is to connect the measured capacitor and a resistor or a current source and use the principle of RC time constant to measure a charging/discharging period, for estimating the capacitance of the measured capacitor. Note that, the capacitor of each trace when the touch panel 100 is not touched is of a very small capacitance around tens to hundreds picofarad (pF). For this reason, when the measured capacitor is an environment capacitor, the charging/discharging period is short, which may result in a large measurement error. Another way to measure the capacitance of the measured capacitor is called charge transfer, which is to transfer electric charges from the measured capacitor to an integrating capacitor of a larger capacitance by one or more than one times until the voltage on the integrating capacitor reaches a predetermined voltage and then discharge the integrating capacitor, to estimate the capacitance of the measured capacitor. Since the method of charge transfer measures the capacitance of the measured capacitance only according to the charging period of the integrating capacitor, measurement is not efficient enough.
Another conventional method, called delta-sigma method, combines the principle of RC time constant and the method of charge transfer. Please refer to FIG. 2, which is a schematic diagram of a capacitance measurement device 20 based on the delta-sigma method according to the prior art. The capacitance measurement device 20 can be used as the capacitance measurement device 104 of the touch control device 10 of FIG. 1. The capacitance measurement device 20 comprises an integrating capacitor 200, a discharging circuit 202, a comparator 204, a digital signal processing unit 206, and switches SW1 and SW2. Please refer to FIG. 3, which is a timing diagram of signals with respect to the capacitance measurement device 20 performing a charging and discharging procedure. FIG. 2 illustrates waveforms of a signal S1 controlling the switch SW1, a signal S2 controlling the switch SW2, the voltage signal VCM on the integrating capacitor 200, and a signal SB outputted from the comparator 204, respectively depicted by a dashed line for the case of a larger capacitance of the measured capacitor 22 and a solid line for the case of a smaller capacitance of the measured capacitor 22.
The signals S1 and S2 respectively control the switches SW1 and SW2 to be turned on at different time. When the switch SW1 is turned off and the switch SW2 is turned on, the voltage source VCC charges the measured capacitor 22; when the switch SW1 is turned on and the switch SW2 is turned off, electric charge stored on the measured capacitor 22 is transferred to the integrating capacitor 200. When charge transfer is ongoing, the comparator 204 compares the voltage level of the voltage signal VCM with a reference voltage VREF and outputs a signal SB as a comparison result. At the same time, the digital signal processing unit 206 converts the signal SB into a count value Dx. When the voltage level of the voltage signal VCM is large than the reference voltage VREF, the signal SB controls the discharging circuit 202 to discharge the integrating capacitor 200. When the measured capacitor 22 is of a large capacitance, electric charge transferred to the integrating capacitor 200 is also a large amount and therefore the duty cycle of the signal SB is high.
Briefly, the capacitance measurement device 20 uses the duty cycle of the signal SB to represent the capacitance of the measured capacitor 22, and converts the signal SB into the digital count value Dx outputted to a rear-stage circuit, so that capacitance variance of the measured capacitor 22 can be determined. Compared to the capacitance measurement by charging/discharging periods or charge transfer previously mentioned, when the measured capacitor is the environment capacitor which is of a small value, the capacitance measurement device 20 obtains a more precise capacitance and has a higher efficiency. However, the capacitance measurement device 20 still has some disadvantages as follows.
The capacitance measurement device 20 uses the measured capacitor 22 of an unknown capacitance to charge the integrating capacitor 200 of a fixed capacitance. In order to estimate touch panels of different characteristics, the capacitance of the integrating capacitor 200 has to be tens of nanofarad (nF), which is far larger larger than the capacitance of the measured capacitor 22 and costs a lot, whatever the integrating capacitor 200 is integrated into an application specified integrated circuit (ASIC) of the capacitance measurement device 20 or is an external component for the capacitance measurement device 20. Moreover, the integrating capacitor 200 is easily interfered with the electromagnetic signals when it is an external component, which may result in instability of the voltage signal VCM on the integrating capacitor 200 and generate noise in the signal SB that influences capacitance measurement accuracy.
In practice, when the discharging period of the integrating capacitor 200 is finished, the voltage level of the voltage signal VCM has to return to an initial voltage level for a next charging period, which intends that the discharging capacity has to be greater than the charging capacity. When the capacitance of the measured capacitor 22 is a large value, electric charge transferred from the measured capacitor 22 to the integrating capacitor 200 is also a large amount. In this situation, if the discharging period for the discharging circuit 202 to discharge the integrating capacitor 200 is not long enough, the voltage signal VCM on the integrating capacitor 200 may have no way to return to the initial voltage level. As a result, the voltage level of the voltage signal VCM accumulates during every charging period. Please refer to FIG. 4, which is a timing diagram of signals with respect to the capacitance measurement device 20 performing a charging and discharging procedure. As shown in FIG. 4, when the discharging period for the discharging circuit 202 is not long enough, the voltage level of the voltage signal VCM accumulates to be the highest voltage level as that of the full-charged measured capacitor 22. In this situation, electric charge stored on the measured capacitor 22 is not transferred to the integrating capacitor 200 and the capacitance measurement device 20 does not work normally. The above problems of the voltage level accumulating may also occur when the charging capacity is greater than the discharging capacity due to environment variance.
When the capacitance of the measured capacitor 22 is far less than the capacitance of the integrating capacitor 200, the voltage level of the voltage signal VCM varies slightly after charge transfer, which intends that the discharging capacity is comparatively larger than the charging capacity. In this situation, it takes more time to charge the integrating capacitor 200 to make the voltage signal VCM reach a voltage level high enough for capacitance measurement. On the other hand, when a short discharging period is used, the capacitance variance of the measured capacitor 22 cannot be measured precisely.
Since the charging or discharging capacity cannot be adjusted in the conventional capacitance measurement devices, the conventional capacitance measurement devices cannot achieve the same measurement accuracy when measuring touch panels of different characteristics. Besides, the conventional capacitance measurement devices and methods cannot renew the environment capacitance. As a result, when a touch panel used for a long time is touched, or a touch panel in an environment with various factors is touched, the rear-stage circuit connected to the conventional capacitance measurement device cannot precisely detect touches since it uses an inaccurate environment capacitance for comparison.